1. Field of the Invention
The present invention relates to a destaging method for a storage apparatus system, as well as to disk control apparatus, a storage apparatus system and a program.
2. Related Background Art
In recent years, the management method used for cache memory in computers has often been the write-back method. This is due to the fact that the write-back method requires less frequent access to disk apparatuses, which can prevent a decline in the processing performance of the computer resulting from disk access penalty. However, since managing the cache memory using the write-back method involves delaying the writing of data onto disk apparatuses, dirty data to be written onto disk apparatuses usually remain in the cache memory.
For this reason, a processing to write onto the disk apparatuses the dirty data remaining in the cache memory (i.e., destaging) must be performed when shutting down the computer.
Destaging must be performed in sequence from upper hierarchy to lower hierarchy in each storage hierarchy within a computer system. This is to make certain that there is no dirty data left in upper hierarchy cache memory. According to conventional technology, the destaging of the lower hierarchy is performed only after the destaging of all upper hierarchy is completed.
However, as the performance of computers continued to improve in recent years, larger capacity cache memories have been mounted on computers, and such cache memories require longer time for destaging.
This becomes a major problem especially when destaging must be performed in an unexpected power outage, for example. This is due to the fact that the time during which power supply is provided by a backup battery mounted on a computer system in power outage situations is limited. If destaging is not completed within the short time during which power is provided by the battery, the dirty data stored in the cache memory will be lost. This can significantly compromise the reliability of the computer system. On the other hand, making certain that the data would not be lost would involve providing a large capacity battery. However, this would not only increase the cost of the computer system, but would also require a large occupying area to mount the battery on the computer.